Conventionally, there has been a structure in which a sensor detecting a rotating speed of each of wheels is provided in a bearing, for a safety drive of a motor vehicle. A conventional general countermeasure for securing a traveling safety of the motor vehicle is taken by detecting a rotating speed of the wheel, however, there is demanded to achieve a further control for a safety by using additional sensor signals other than speed sensor signals because only the rotating speed of the wheel is not sufficient.
Accordingly, there can be considered to achieve a posture control based on the load applied to each of the wheels during travel of the vehicle, since the load applied to each of the wheels is not uniform. For example, a great load is applied to the outside wheels in a cornering, the load is biased to the one side wheels during travel on a lateral slope surface, and the load is biased to the front wheels at a time of braking. Further, in the case that a superimposed load is uneven, the applied load becomes uneven. Accordingly, if it is possible to detect the load applied to the wheels any appropriate timing, it is possible to carry out a posture control at a time when the vehicle travels (a rolling prevention at a time of cornering, a front wheel sinking prevention at a time of braking, a sinking prevention caused by an unevenness of the superimposed load or the like) by controlling a suspension or the like in advance on the basis of results of detection. However, since there is no suitable installing place for a sensor detecting the load applied to the wheel, it is hard to achieve the posture control on the basis of the load detection.
Also, in the event that in the future the steer-by-wire is introduced to provide a system in which an axle and a steering wheel come not to be mechanically coupled, it will be demanded to detect a load acting in an axial direction of the wheel so as to transmit information on the road surface to the steering wheel held by a driver.
In order to meet the demand mentioned above, there has been proposed a bearing for a wheel with a strain gauge attached to an outer race of the bearing for the wheel so as to detect the strain acting on the outer race, for example, in Japanese Laid-open Patent Publication No. 2003-530565.
The outer race of the bearing for the wheel has at least one rolling contact surface and therefore should have a sufficient strength. Also, since it is manufactured through a complicated process such as plastic working, turning, heat treatment and grinding, the structure in which the strain gauge is attached to the outer race as shown in the publication mentioned above, lead to a low productivity and a high cost in case of mass production.
In order to solve the above problem, it was attempted to attach a strain sensor to a sensor fitting member so as to form a sensor unit, and to attach the sensor unit to the outer race. In this case, if it costs too much to manufacture the sensor fitting member, it is impossible to achieve a cost reduction. Accordingly, it is desired to achieve the cost reduction at a time of mass production by making the sensor fitting member be easily worked.
Further, if the strain gauge is attached to the outer race so as to be exposed as described in the above publication, the strain gauge is often exposed to a corrosive gas or a corrosive liquid, and as a result, the strain gauge is corroded, thereby failing to carry out an accurate sensing. The bearing for the wheel is generally under a severe condition that it is exposed to a road surface and comes under a salty muddy water. Even in the case that the strain gauge is provided in a sealed bearing space, the corrosive gas or the corrosive liquid occasionally intrudes thereto and accordingly, it is impossible to completely prevent the corrosion of the strain gauge.
Further, an output signal of the load detecting sensor such as the strain gauge is transmitted, for example, to a sensor signal processing circuit provided in an electric control unit of a motor vehicle so as to be utilized for calculating an external force acting on the bearing for the wheel and an acting force between a tire and the road surface or the like as well as for controlling the motor vehicle. However, conventionally a wiring connecting between the load detecting sensor and the sensor signal processing circuit is complicated with the consequence of no-easy wiring work.